PROPOSAL SUMMARY The intestinal epithelium protects against food antigens and the luminal microbiota by forming a physical barrier and actively secreting mucous and anti-microbial peptides. The intestinal epithelium is maintained by a process of constant renewal through a cycle of cell division, differentiation, and apoptosis. Apoptosis is a potent inducer of regulatory CD4 T (TREG) cells and immune tolerance. During infection, apoptosis induces a tailored CD4 T helper 17 (TH17) response. Whether apoptosis of intestinal epithelial cells (IEC) plays any role in intestinal tolerance and effector response is not known. Professional mononuclear phagocytes (MP), such as macrophages and dendritic cells (DC), clear microbial pathogens during infection and apoptotic cells during embryonic development and tissue remodeling. While much attention has centered on MP sampling of commensal and pathogenic microorganisms within the gut lumen, less attention is directed towards sampling of apoptotic IEC, despite the prominent role that apoptosis plays in the intestinal epithelium. Interference with the natural cycle of apoptosis in IEC leads to intestinal inflammation in mouse models. There is increased IEC death and damage to the intestinal epithelium in patients with inflammatory bowel disease (IBD), and this fuels intestinal dybsiosis and inflammation. Several international IBD genome wide association studies (GWAS) and meta-analyses have associated susceptibility to IBD with genes involved in pathways regulating innate immune responses, immunomodulatory cytokines, and TH17 signaling. Gene expression data from different murine cell types have shown the strongest enrichment of IBD genes in innate immune cells, particularly DC. Gaining an understanding of whether and how apoptosis of the intestinal epithelium impacts the processes of tolerance and immunity within the intestine is therefore of utmost importance. Using two novel mouse models, we will determine how increasing or impairing apoptosis of the intestinal epithelium impacts the homeostatic and immune responses within the intestine. We have compelling evidence that apoptosis in the small intestinal epithelium is not a bystander event, but actively imprints lamina propria MP with `suppression of inflammation' and `induction of TREG cell' transcriptional signatures. Notably, many of the genes differentially modulated in MP after apoptotic IEC uptake overlap with IBD genes. Here we will examine the identities and profiles of colonic MP during steady state and infection. We will investigate how apoptosis impacts the functions of intestinal MP as they relate to various mechanisms of immune tolerance and effector response in the intestine. This includes their response to microbial components, their maintenance of innate lymphoid cells, and their instruction of TREG and TH17 cell fates. All these parameters are disrupted in IBD. By the completion of these studies, we will have gained new insights into the role of apoptosis in intestinal immune regulation, advance our understanding of how homeostasis is maintained within the mucosa, and set the stage for development of novel therapeutics for inflammatory diseases such as IBD.